The present invention relates to a reciprocating type oil-free gas compressor.
Generally, reciprocating compressors use lubricating oil between the piston and the cylinder, but where a gas compressor is used as a helium compressor for use with a helium liquefying apparatus, it is required that no lubricating oil be used, since if helium is contaminated with lubricating oil, this would result in icing, which is undesirable from the standpoint of production of cryogenic temperatures. Such gas compressor is generally called the oil-free gas compressor.
For prevention of oil creep-up, conventional oil-free gas compressors use a rod packing installed on the piston rod reciprocated by a cross head or the like or use other oil creep-up preventer, thereby preventing the lubricating oil from creeping up the piston rod into the cylinder to contaminate, e.g., helium gas.
Conventionally, where helium or other expensive gas is to be compressed, another cylinder or a chamber called the distance room is interposed between the cylinder in which the piston is reciprocated and the crank chamber in which the cross head for reciprocating the piston is housed, so that the gas leaking from the cylinder is recovered in such chamber, thereby preventing the gas from leaking out of the compressor.
For example, a conventional gas compressor having a rod packing is as schematically shown in FIG. 1. A piston rod 2 for reciprocating a piston 1 is connected at its lower end to a cross head 5 through a rod packing 3 of Teflon or graphite carbon and an oil seal 4. The space between the rod packing 3 and the oil seal 4 defines a distance room 6. The rod packing 3 comprises a plurality of packing rings 9 housed in a packing box 7 and held by a packing gland 8. In addition, the numeral 10 denotes a crank rod which performs a crank motion. Thus, the movement of the crank rod 10 causes a vertical reciprocating movement of the cross head 5, which, in turn, causes a reciprocating movement of the piston through the intermediary of the piston rod 2, thereby compressing the gas in the cylinder. The lubricating oil in mist form fills the cylinder almost up to the oil seal 4, but it is blocked by the oil seal 4 and the oil adhered to the piston rod 2 is thrown away by the rod packing 3, so that no oil creeps up to the back of the piston 1.
Thus, the conventional reciprocating type oil-free gas compressor employs a construction using a large number of parts for prevention of oil creep-up, and furthermore, some amount of gas is leaking from the back of the piston into the distance room 6. As for the leakage gas, there is no alternative but to recover it by a special recovering device or to let it out into the atmosphere. Thus, there has been much to worry about measures against leakage gas.
Accordingly, an object of the present invention is to provide an oil-free gas compressor simple in construction and capable of recovering leakage gas from the piston without allowing said gas to mix with the lubricating oil, thereby solving the problems with the prior art.
Another object of the invention is to prevent leakage of gas from the compression piston to avoid economic loss.
A further object of the invention is to provide an arrangement wherein a piston and a guide piston are oil-tightly separated from each other by a single diaphragm while giving a minimum of tension to said diaphragm.